1. Field of the Invention
The present invention relates to a signal transmission circuit, and particularly to a signal transmission circuit including a common mode filter and a capacitive element.
2. Description of Related Art
Conventionally, a differential transmission method has been used for transferring digital signals between electronic devices. Differential transmission is a method of inputting a digital signal into a pair of signal line paths in reverse phase so that radiation noise produced from the signal lines and external noise can be cancelled. Since external noise is cancelled, total noise is also reduced. Since signals are not affect by noise, signals can be transmitted at smaller amplitude. Therefore, the rise and fall time of the signal is shortened, enabling high-speed signal transmission.
Types of cable capable of performing the differential transmission method include USB, DVI, and the more recent HDMI type capable of transmitting more digital signals. HDMI cable uses a high-speed serial interface for connecting a digital transmitter, such as a DVD player or a set top box, to a digital television or the like for transmitting uncompressed digital signals. One HDMI cable is capable of transferring both of video signals and voice signals at a high speed.
When increasing the signal transmission speed, noise can be generated by a slight offset in the differential signal between signal lines. One transmission circuit proposed in Japanese patent application publication No. 2001-85118 reduces noise by inserting a common mode choke coil in a cable or other interface.
However, the impedance on the transmission path must be matched when high-frequency or high-speed pulse signals are transmitted. If the impedance is not matched, the high-frequency component of the signal is reflected by the unmatched part of the impedance, resulting in a large return loss that greatly attenuates the signal. Further, unnecessary radiation noise is produced by the reflection.
A capacitive element such as a varistor, capacitor, or zener diode is normally inserted into the connection part of the digital television with the interface as an antistatic component. The graph in FIG. 1 shows characteristic impedance for positions on the transmission path when a capacitive element (1.1 pF) is inserted on the transmission path and when a capacitive element is not inserted on the transmission path. Normally, as capacitance component increases, impedance reduces. It would be seen that the capacitance component reduces the characteristic impedance at the location of the antistatic component in FIG. 1. As a result, the high-frequency component of the signal is reflected at the position of the antistatic component, generating noise.